[skip ci] Reference about store_groupchat_messages option in mod_offline added

[antivista]

This PR adds documentation for mod_offline, regarding the option store_groupchat_messages. #2879

[DenysGonchar]

Thanks for PR. historically there are several reasons why store_groupchat_messages option is not documented:

1. XEP-0160 doesn't specify this functionality, because the classic MUC even doesn't send the messages to offline users.
2. As this will not work as expected with the classic MUC, the option makes sense only with MUC-light.
3. XEP-0160 is not recommended way of storing offline messages (especially group-chat messages), better use message archive with MUC storage enabled.

So this option is rather confusing than brings any benefit, it's kept for compatibility reasons.

So let's either keep it not documented or fully document it.

[antivista]

Although the use of mod_mam is the better approach in most of the cases, client side requires more effort (compared with XEP-0160) to implement XEP-0313, so I have thought that to document this option could be good in these scenarios:

• As a momentary alternative waiting the XEP-0313 implementation client side;
• If it is expected the use of just one client per user and when a message is delivered (for some reason) we do not want allow the rescue of that message again, to do this with mod_mam should require more work because XEP-0313 does not foresee a mechanism of message deletion except for the older messages in the archive.

If these scenarios are considered of interest, I can try to fully document, referring to explicitly to MUC-light case as DenysGonchar rightly pointed out to me.

[antivista]

I hope that it is adequately documented now.

[NelsonVides]

Fine by me 👍🏽

[DenysGonchar]

👍 thanks for your contribution